I am trying to design a replacement deck for my home. Local regulations require IRC 2015 and the area is designated SDC D2. The surface height of my deck will be ~8' 8" above grade and there is a cement patio under the deck.
In DCA6 page 11, it shows that "Joist on Beam" and "Joist hung from Beam" are allowed. I'm trying to preserve as much headroom on the patio as possible and have chosen to hang the joists. Do the top of the joists have to lay in the same horizontal plane as the top of the beams as shown in DCA6 or can I set the joists lower so the decking will line up with the top of the beams?
Using the middle figure concerns me that water and moisture retention may cause rotting where the decking lies on top of the beam. Are there other concerns in leaving the beam exposed as decking surface?
Best Answer
Yes, by code you can raise the top of the beam to be at the top of the decking. However, 1) it will give you a “pattern” on your deck that you probably won’t like, 2) you’ll need to strap the two sides together, because you’re relying on withdrawal to keep the deck together. Usually we run the decking across the beams and then the decking acts as a “strap” too.
Also, having the top of your “built-up” beam exposed will allow moisture to lay between the members and cause dryrot. Having the deck on top of you “built-up” beam will “protect” the members and keep moisture out.
Where you live with seismic D2 requirements, you’ll need diagonal braces (especially that high in the air) and the code now requires a minimum of 2 hangers like this to attach to the house:
http://embed.widencdn.net/pdf/plus/ssttoolbox/sonpfljoja/F-DECKCODE17-p019.pdf
Also, consider using stainless steel hangers with either pressure treated or cedar or redwood lumber for longevity.